The present invention relates to a spin coating method and a coating apparatus for forming a thin film having a uniform thickness on a substrate in a process for manufacturing, for example, semiconductors, optical disks and the like.
The techniques for forming a thin film having a uniform thickness on a substrate is very important in various fields. For example, upon coating a resist or a reflection preventing solution on a wafer in a semiconductor manufacturing process, or upon coating a light-absorbing dye solution etc. on a disk in an optical disk manufacturing process, it is very important that these chemical solutions are coated uniformly to form a thin film having a uniform thickness.
As a method for forming a thin film having a uniform thickness, one example thereof is a spin coating method. This method is a method in which a coating solution is dropped from a nozzle onto a substrate in the stationary or turning state and, thereafter, this substrate is turned at a high speed to remove the unnecessary solution to form a thin film having a uniform thickness on the substrate surface.
According to this spin coating method, during waiting in a nozzle after dropping a coating solution from a nozzle and until dropping again the coating solution next time, since a tip of the nozzle contacts with an outer air, a part of a solvent in the coating solution evaporates and the composition of the coating solution contained in the nozzle tip is changed. This causes a problem that the ununiformity of the thin film thickness occurs upon forming a thin film.
In particular, when a solute having high crystallizability such as dye and the like is contained in the coating solution, during waiting in the nozzle, crystals are deposited at the nozzle tip and the crystals are fallen on the substrate together with the coating solution upon dropping the solution, leading to defects derived from the crystals in some cases.
In order to suppress this, the provision of a washing mechanism at the nozzle tip is generally carried out. However, this not only makes an apparatus large scale but also the use of a solvent as a washing solution changes the concentration of the coating solution because of the remaining solvent at the nozzle tip, which also leads to the ununiform thin film thickness.
When a solution having a composition near to that of the coating solution containing a solute is used as a washing solution, the aforementioned disadvantage in the case of the solvent alone is avoided but, where the cost of the coating solution used is high, the use of a large amount of the washing solution leads to the cost-up, resulting in not practical use.
In addition, for example, a method is disclosed in JP-A-8-168714, in which an initial portion of a coating solution which has been dropped is recovered and only the solution thereafter is dropped onto a substrate. According to this method, when a solute having high crystallizability is contained in a coating solution, the deposited crystals are removed to a some degree but this is still not complete. In addition, like the nozzle washing method as described above, since a part of the coating solution is lost, the cost-up is caused.
Further, for example, there is described the change in the shape of a nozzle tip and prevention of solution falling from the nozzle tip therefrom in JP-A-7-321029. However, the change in the shape of the nozzle tip dose not prevent the crystals from deposition.
Therefore, an object of the present invention is to overcome the problems of the prior art and provide a spin coating method and a coating apparatus for forming a thin film having a uniform thickness on a substrate, at a low cost, in a process for manufacturing, for example, semiconductors, optical disks and the like.
Inter alia, an object of the present invention is to provide a spin coating method and an apparatus for forming a thin film having a uniform thickness on a substrate, at a low cost, without the deposition of crystals at a nozzle tip, even when a solution containing a solute having high crystallizability is coated.
The present inventor intensively studied a method which can suppress the deposition of crystals at a nozzle tip even when a solution containing a solute having high crystallizability is coated.
First, it is contemplated that a nozzle tip is dipped in the same solvent as that of a coating solution during waiting in the nozzle. However, in this method, a solvent is attached to the nozzle tip, the concentration of the coating solution immediately after initiation of dropping changes and, for this reason, the coating nonuniformity occurs in some cases.
The present inventor found that, in order to solve this, it is effective to soak the nozzle tip in a solution having a composition near that of a coating solution in waiting in a nozzle, which resulted in the completion of the present invention.
That is, the present invention is a spin coating method for forming a thin film on the surface of a substrate by dropping a coating solution containing at least one solute and a solvent from a discharge nozzle onto the surface of a substrate to be coated, mounted on a horizontal turn table and turning the substrate, said method comprising soaking and holding a nozzle tip in a nozzle soaking solution containing at least one of said solute and the same solvent as said solvent or a solvent having a similar chemical structure to that of said solvent, after the coating solution is dropped from the nozzle.
In the present invention, it is preferred that the nozzle soaking solution is a solution containing at least one of said solute and the same solvent as said solvent.
In the present invention, it is preferred that the nozzle soaking solution is a solution having a composition near that of the coating solution.
In the present invention, it is preferred that the nozzle soaking solution contains a main component of the solute of the coating solution, and the concentration of the main component in the nozzle soaking solution is 10 to 200% relative to the main component concentration in the coating solution.
In the present invention, it is preferred that the nozzle soaking solution contains all the components of the solute of the coating solution, and the concentration of each of these components in the nozzle soaking solution is respectively 10 to 200% relative to the concentration of each component in the coating solution.
In the present invention, the nozzle soaking solution is a solution having the same composition as that of the coating solution.
In addition, the present invention is a spin coating apparatus comprising a horizontal turn table on which a substrate to be coated is mounted and which turns the substrate and a discharge nozzle for dropping a coating solution onto the surface of a substrate, said apparatus comprising a nozzle soaking means for soaking and holding a nozzle tip after dropping of the coating solution in a nozzle soaking solution.